MONDAY, Dec. 22, 2014 (HealthDay News) -- If you want to battle belly fat as you age, new research suggests you need to add weight training to your exercise regimen.

Researchers from the Harvard School of Public Health found that combining aerobic activities with weight, or resistance, training is key to preserving muscle and avoiding weight gain, particularly age-related belly fat.

"Because aging is associated with sarcopenia, the loss of skeletal muscle mass, relying on body weight alone is insufficient for the study of healthy aging," study author Rania Mekary, a researcher at Harvard's department of nutrition, said in a university news release.

The long-term study was conducted between 1996 and 2008. It included more than 10,000 healthy men aged 40 or older whose body mass indexes (BMI) varied widely. BMI measures body fat by looking at weight and height.

The researchers analyzed the men's physical activity, weight and waist circumference to determine which exercises had the most significant effect on the men's waistlines, or the amount of belly fat they had.

The men who did 20 minutes of weight training daily had a smaller increase in belly fat than the men who spent the same amount of time engaging in moderate to vigorous aerobic activities, such as stair climbing and yard work, the study, published online Dec. 22 in the journal Obesity, found.

Meanwhile, the men who became more sedentary over the course of the 12-year study had a larger increase in belly fat.

"This study underscores the importance of weight training in reducing abdominal obesity, especially among the elderly," study senior author Frank Hu, a professor of nutrition and epidemiology, said in the Harvard news release. "To maintain a healthy weight and waistline, it is critical to incorporate weight training with aerobic exercise."